The Merk turret is actually on an M1 hull.Both the Sheridan and the Bradley (M113 also) were designed with aluminum hulls. It was to make them lighter for amphibious operations more than for armor protection. Even then, the Bradley was almost 10 tons heavier when it entered service. I believe it's now about 15 tons heavier.

Funny story, when Australia was upgrading their ancient M-113A1s they went for a hull stretch. All well and god but they discovered many (most) of the aluminium hulls had delaminated, which made them painful to fit in the jigs, let alone weld. Ironically though it was discovered that this delamination, far from weakening the vehicles structure, as feared, had actually increased their resistance to penetration.